This invention relates to low inductance probes, and more particularly to low inductance probes in which the distance between the signal probe point and the ground probe point is adjustable.
A typical prior art probe 10 is shown in FIG. 1. A signal is measured with a signal probe consisting of signal probe point 12, which is insulated by insulator 14, and shielded from electromagnetic interference by conductive shield 16. A signal conductor (not shown) is in electrical contact with the signal probe point 12 and extends the length of the shield 16 and into the probe grip 22. A ground probe consists of ground probe point 18 which is used to contact a circuit ground location and establish a reference for the measured signal. The ground probe point is insulated by insulator 20. A ground conductor (not shown) is in electrical contact with the ground probe point and extends the length of insulator 20 into the probe grip 22. Inside the probe grip 22, the ground conductor is attached to the shield 16. The signal conductor and the shield 16 form a shielded cable 26. A cable guard 24 may be used to prevent excessive wear at the point which the shielded cable 26 exits the probe grip 22. Although the prior art probe 10 can be made to have relatively low inductance in series with the ground probe point 18, the distance between ground probe point 18 and signal probe point 12 is fixed. Thus, prior art probe 10 cannot be used to make measurements where the desired measuring location is other than a fixed distance from a convenient ground location.
Another prior art probe 28 is shown in FIG. 2. As in the probe of FIG. 1, probe 28 contains a signal probe consisting of a signal probe point 12, an insulator 14, and a conductive shield 16. Similarly, a ground probe known as a "Z" lead consists of ground probe point 18' which is used to contact a circuit ground location and establish a reference for the measured signal insulated by insulator 20'. However, in probe 28, the ground probe not only makes electrical contact with shield 16, but is free to rotate along its axis with probe grip 22. Therefore, the distance between the signal probe point 12 and ground probe point 18' is not fixed but can be set to various points determined by the amount that the ground probe is rotated away from its center axis. The prior art probe 28 can also be made to have relatively low inductance in series with the ground probe point 18'. However, due to the freedom of movement of the ground probe, in actual use prior art probe 28 is hard to manipulate, especially with one hand. Also, the range of distances between the signal probe point 12 and the ground probe point 18' is limited. Thus, prior art probe 28 cannot be conveniently used to make measurements and cannot make measurements at all where the desired measuring location is far from a convenient ground location.
What is desired is a low inductance probe where the distance between the signal probe point and the ground probe point is adjustable over a wide range, and measurements can be made conveniently using one hand.